A good deal of computer and electronic equipment, such as free convection cooled electronic telephone switching equipment, becomes temperature sensitive with age. In the case of central office telephone equipment, this problem has resulted in several significant telephone network outages which have been attributed to thermal shock, i.e., when the rate of temperature change in the central office is too great. One specific situation in which this occurs is when, after an outage, the air conditioning system comes back on line, such as when power is restored to the cooling system after a power failure.
In many existing telephone central office building system designs, cooling systems are designed for high velocity air movement that is directed towards equipment cabinets or bays. This type of design is intended to allow air to be moved around extensive overhead air blockages, such as cable distribution systems normally found in central offices. Cooled air is directed against equipment with the highest heat dissipation. If the air conditioning fails, the temperature of the equipment rises gradually with ambient temperature in the central office. When the air conditioning is thereafter restored, cool air is directed toward the electronic equipment, causing a very fast change in temperature. This type of thermal stressing or thermal shock causes failures in the electronics to occur.
To avoid the problems just described, certain maintenance procedures have been attempted to keep room temperature variation to a prescribed level, and to keep temperature changes within a desired tolerance, such as 10 degrees/hr. Procedures that require manually shutting off dampers, starting one of multiple cooling fans at a time, or manually switching the cooling system on and off have been recommended and attempted. In all cases, these procedures have failed because of the inability of a human to monitor and accurately control the system. Because of this human element and the varied and complex nature of these building systems, manually controlling temperature variation has proven hard to implement. It has not solved the problem of directed cool air causing fast increases in heat transfer rates of electronic devices.
Another current approach involves the use of heating, ventilation and air conditioning (HVAC) systems designed with low velocity horizontal air movement. The purpose of such systems is to allow rapid temperature changes to be minimized. This system, which is used, for example, on new raised floor installations, gives a certain thermal inertia to the office, by not blowing air directly on equipment. However, this approach is neither cost effective nor risk averse for existing installations, because replacement of existing high velocity air delivery systems is very expensive and can adversely affect electronics.